Probably because of slow mirrors. That’s a real problem with the installer, it doesn’t let you change mirrors, it picks the nearest one or something like that, which could be a very bad mirror. For example, I live in Argentina and the installer often picks the La Plata University mirror which has had very unstable performance for some months now. Installing 19.04 (which I did about 5 times in June) could take anything from 15 minutes to 2 hours, depending on the day. One workaround is to disable updates during installation and do them post-installation after picking a better mirror. But the installer should provide an option to select mirrors or perform some analysis of actual mirror speed.